diff --git a/handin-server/checker.rkt b/handin-server/checker.rkt
index a549c56..ae739ea 100644
--- a/handin-server/checker.rkt
+++ b/handin-server/checker.rkt
@@ -327,7 +327,9 @@
 
 ;; for adding lines in the checker
 (define added-lines (make-thread-cell #f))
-(provide add-header-line!)
+(provide add-header-line! line:empty line:unsubst)
+(struct line:empty () #:prefab)
+(struct line:unsubst (str) #:prefab)
 (define (add-header-line! line)
   (let ([new (list line)] [cur (thread-cell-ref added-lines)])
     (if cur
@@ -482,6 +484,13 @@
                    (define generic-substs `(("submission" . ,submission-dir)))
                    (define (prefix-line/substs str)
                      (prefix-line (subst str generic-substs)))
+                   (define (prefix-line/substs* str)
+                     (if (line:empty? str)
+                       (newline)
+                       (prefix-line
+                        (cond [(string? str) (subst str generic-substs)]
+                              [(line:unsubst? str) (line:unsubst-str str)]
+                              [else (error* "bad value in prefix lines: ~e" str)]))))
                    (define (write-text)
                      (set-run-status "creating text file")
                      (with-output-to-file text-file #:exists 'truncate
@@ -489,7 +498,7 @@
                          (for ([user (in-list users)])
                            (prefix-line (user-substs user student-line)))
                          (for-each prefix-line/substs extra-lines)
-                         (for-each prefix-line/substs
+                         (for-each prefix-line/substs*
                                    (cond [(thread-cell-ref added-lines)
                                           => unbox]
                                          [else '()]))
diff --git a/handin-server/scribblings/checker.scrbl b/handin-server/scribblings/checker.scrbl
index f379f37..60ad1bf 100644
--- a/handin-server/scribblings/checker.scrbl
+++ b/handin-server/scribblings/checker.scrbl
@@ -205,7 +205,7 @@ Keywords for configuring @racket[check:]:
   @racketblock[
     (lambda (msg)
       (add-header-line! "Erroneous submission!")
-      (add-header-line! (format "  --> ~a" msg))
+      (add-header-line! (line:unsubst (format "  --> ~a" msg)))
       (message (string-append
                 "You have an error in your program -- please hit"
                 " \"Run\" and debug your code.\n"
@@ -332,12 +332,21 @@ code.}
   submission directory, then the system will not allow ``@tt{foo}'' to
   submit with ``@tt{bar}''.)}
 
-@defproc[(add-header-line! [line string?]) void?]{
+@defproc[(add-header-line! [line (or/c string? line:empty? line:unsubst?)]) void?]{
   During the checker operation, can be used to add header lines to the
   text version of the submitted file (in addition to the
   @racket[:extra-lines] setting).  It will not have an effect if
   @racket[:create-text?] is false.}
 
+@defproc[(line:empty) line:empty?]{
+  Helper for @racket[add-header-line!]: creates a value that results in an
+  added ampty line (not prefixed).}
+
+@defproc[(line:unsubst [str string?]) line:unsubst?]{
+  Helper for @racket[add-header-line!]: creates a value that results in a
+  prefixed line with the given text which is not subtituted via [user-substs].
+  Use this to avoid erroneous values from user strings (like error messages).}
+
 @defproc[(procedure/arity? [proc procedure?] [arity number?])
          boolean?]{
   Returns @racket[#t] if @racket[proc] is a procedure that accepts